variants oftenO
plural o's or osˈōz 
often attributive
1
a
:the 15th letter of the English alphabet
b
:a graphic representation of this letter
c
:a speech counterpart of orthographico
2
:a graphic device for reproducing the lettero
3
:one designatedo especially as the 15th in order or class
4
:something shaped like the letter O
especially:zero
1
ocean
2
ohm
3
old
4
order
5
over
less common spelling ofoh
1
—used to express an emotion (such as surprise or desire) or in response to physical stimuli
Oh, no! I forgot my key.
Oh, don't stop!
2
—used in direct address
Oh, waiter! Will you come here, please?
3
—used to express acknowledgment or understanding of a statement
Oh, yes. It's one of my favorite cities.
4
—used to introduce an example or approximation
Their house is about,oh, I'd say four miles from here.
:the one of the four ABO blood groups characterized by the absence of antigens designated by the letters A and B and by the presence of antibodies against these antigens
1
offense
2
Ohio
3
[New Latinoctarius]pint
oxygen
orth-or ortho-
-o-
9 of 11affix
—used as a connective vowel originally to join word elements of Greek origin and now also to join word elements of Latin or other origin
speedometer
elastomer
:one that is, has the qualities of, or is associated with
bucko
-o
11 of 11interjection suffix
—in interjections formed from other parts of speech
cheerio
righto
Examples ofo in a Sentence
Noun (1) a word that starts with ano a word that starts witho
Word History
Etymology
Combining form
Greekōi-, ōio-, fromōion — more ateggentry 1
Affix
Greek, thematic vowel of many nouns and adjectives in combination
Noun suffix
perhaps fromohentry 1
Interjection suffix
probably fromohentry 1
First Known Use
Noun (1)
before the 12th century, in the meaning defined atsense 1a
Noun (2)
1929, in the meaning definedabove
Time Traveler
The first known use ofo was before the 12th centuryPhrases Containingo
Browse Nearby Words
-o
o
-o-
Cite this Entry
“O.”Merriam-Webster.com Dictionary, Merriam-Webster, https://www.merriam-webster.com/dictionary/o. Accessed 16 Dec. 2025.
Copy CitationKids Definition
O
2 of 2variant ofoh entry 1
Medical Definition
o-
1 of 4abbreviationorth-; ortho-
O
2 of 4noun:the one of the fourABO blood groups characterized by the absence of antigens designated by the letters A and B and by the presence of antibodies against these antigens
O
3 of 4abbreviation1
opening
2
[Latinoctarius]pint—used in writing prescriptions
O
4 of 4symboloxygen
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